An LTE-U deployment can use an unlicensed spectrum that is today generally used by Wi-Fi. Typically, LTE-U refers to a stand-alone version of Long-Term Evolution (LTE) operating in the unlicensed band. License-Assisted Access via LTE (LAA-LTE) is one of the main work items for the 3GPP LTE Release 13 standard being proposed as a technology for operation on both a licensed and an unlicensed spectrum.
In an LAA-LTE deployment, the User Equipment (UE) connects to an LTE network on a regular, licensed spectrum band, the so-called primary cell (P-cell). In addition, it can also connect to the same network on an unlicensed spectrum, i.e., the secondary cell (S-cell). There may be more than one S-cell, i.e., more than one carrier in the unlicensed band. The unlicensed carrier can be used to relieve the licensed carrier from data whenever it is available. In this way the licensed carrier can be used for robust control signaling and the unlicensed carrier can be used to boost user-data rates in a best-effort fashion.
One of the main goals of the 3GPP work item is to find a solution to handle coexistence with other access technologies, such as Wi-Fi, and also between several LAA-LTE systems.
Listen-Before-Talk (LBT) is a protocol by which the desired channel on the wireless medium is first sensed for any potentially interfering transmissions before a transmission begins. If the medium is found to be free, the transmitter can start using it. Together with a back-off mechanism an LBT-protocol potentially avoids collisions. See, for example, the Load-Based Equipment (LBE) protocol described in ETSI EN 301 893.
The LBT protocols usually comprise a number of steps that include the step of listening to the medium, e.g. by measuring the received signal with procedures such as Clear-Channel Assessment (CCA), the step of deciding if the medium is busy/free, e.g. based on energy detection and/or decoding of signal, and the step of starting the transmission, e.g. if medium is free, or after a defined back-off period, the system starts the transmission.
Although LBT is used there is a possibility that different transmitters start their transmissions at the same time causing a collision and interference to each other. The way an LTE-U system handles this collisions is implementation-dependent. One way to handle collisions is like the standard LTE system handles disturbances of transmissions, i.e., the receiver either does not respond or sends a negative acknowledgement.
When transmitting signals on a transmission channel a receiving path of a communication device is turned off when the transmitting path is turned on since otherwise the receiving path would receive the transmission of the transmitting path.